Efficacy of Metarhizium Anisopliae icipe 78 and Acaricides in the Management of the two-Spotted Spider Mite, Tetranychus Urticae (Acari: Tetranychidae), on French Beans, (Phaseolus Vulgaris l.)

Abstract

French bean, (Phaseolus vulgaris) L. is key commodity for export in Kenya. The value of export increased from 4,466.7 Kenya Shillings in 2002 to 4,466.7 Kenya Shillings in 2006. Production of French beans is constrained by many factors including pests, lack of capital and credit facilities, fluctuation in demand, and difficult accessibility to the market, and improper grading. The major arthropod insect pests of French beans include bean stem maggots, flower thrips, pollen beetles, foliage beetles, and Tetranychid mites, Tetranychus urticae Koch, in particular. Emphasis has been put in breeding lines for resistance and developing IPM for the major insect pests of French beans, but little has been done in developing cost-effective, environmentally safe management strategy for the two-spotted spider mite, Tetranychus urticae. The fungal pathogen Metarhizium anisopliae (Metschnikotl) Sorokin has been considered a potential candidate for the control of Tetranychid mites. including the two-spotted spider mite. To be incorporated in the French bean production system, Metarhizium anisopliae has to be compatible with the pesticides used for the control of other pests and diseases. The in vitro compatibility of the entomopathogenic fungus Metarhizium anisopliae with 4 acaricide formulations was evaluated in order to incorporate both in the control of twospotted spider mite. The active ingredients Abamectin, Propargite, Fenpyroximate and Sulphur were tested in three different concentrations: manufacturer's recommended field rate, ten times less recommended rate and hundred times less recommended field rate by incorporating them into SDA media. The effect of acaricides on conidia germination and vegetative growth was compared. The formulations tested affected conidial germination and vegetative growth of the fungus in different levels. A stimulatory effect was observed with Abamectin on conidial germination. The results indicated that propargite, fenpyroximate and sulphur are not compatible with Metarhizium anisopliae because they caused strong inhibition in its development. The compatible formulation with Metarhizium anisopliae was Abamectin. A laboratory study was conducted to evaluate the effects of low rate of Abamectin on the pathogenicity of Metarhizium anisopliae against T. urticae by transferring adult female mites to leaf discs treated with fungal isolate or acaricide or in combination. Synergism between the fungus and the acaricide was not observed and their combination was as effective as the fungus alone on the mortality of mites. Conclusively, combined application of the fungal formulation with abamectin at the low rate is not economical thus not a potential alternative for sustainable control of the mites. The efficacy oj Metarhizium anisopliae and low application rate of Abamectin on T. urticae was studied under field and semi-field conditions. Mite density declines and relative efficacy of treatments were determined by sampling all plots at 7 days interval. Metarhizium anisopliae reduced the population density of mites as compared to Abamectin and the controls. Fungus-treated plots gave better yields, number of pods/plant and were slightly damaged as compared to Abamectin plots and untreated controls. This study indicates that Metarhizium anisopliae hold the potential for the practical management of T. urticae.